Kitchen with Ultra Fin PEX added to 80 % Boiler System Suggstions

[buffalodoug]

Hi

I have my remodeled kitchen with full underfloor access below. I have installed ultrafin with pex 2 1/2 inches below the subfloor as recommended.

My problem is my Old 80% Boiler with one zone and cast iron radiators throughout for my 2000 s/f home. I have tried a tie in to one "Return" leg from a radiator to the new pex (for the lower temp of the return) but for some reason, the water is not getting through this new connection. The pex just does not get hot. I have purged air from the pex and from the cast iron too, but still the hot water does not get through the pex.

I know this is not an idea set up. Can anyone suggest a better solution here without buying a new furnace? I am open to a set up that would heat exchange from a hot water tank, but could use some diagrams and parts list to get it done.

Thanks in advance.

Doug

[Tom Sawyer]

It ain't the boiler, it's the piping. You can't just bug a radiant run onto existing piping or manifolds and expect to get good results. As you have seen, you sometimes get no results at all. That's because there is friction involved in that hundred foot or so of pex under there and water being fairly stupid, will always flow to the path of least resistance which unfortunately in your case, is not your radiant loop. Besides that bit of aggravation, there is also the issue of delta T across the loop to consider and that usually requires some sort of 3way zone valve, a separate circulator and primary secondary piping or a hydronic separator which is what I have been going to lately along with VFD pumping which takes the guess work out of things. So, the good news is that you have the piping installed already. The bad news is that you are going to need to do a total re-pipe of the boiler and existing zones as well as make some decisions as to control strategy because a normal thermostat will overheat the hell out of that zone also. A floor temp sensor is a better idea.

[BadgerBoilerMN]

Perhaps you need a real hydronic designer? Or you can go with Huck. Either way, DIY radiant design is always the first mistake.

[Tom Sawyer]

Probably get a nasty reply, yet again for suggesting such things should not be attempted by neophytes but I'm used to it by now.

[Dana]

Some DIYers can (and will) do the math, most don't.

There are wealth of websites out there implying that it's dead-simple and that it can be simply hacked, but experience proves otherwise. Paying a pro to "fix" a poorly implmented hack can be more expensive than letting somebody with the design experience & tools have-at-it from the start.

There are a number of ways to run radiant off a cast-iron boiler, but the design-by-webforum approach won't necessarily provide the most direct or cheapest path to success.

You started with a single-zone system, and there's no way to reliably run the whole system as a balanced single zone with the radiant running as a passive side-loop off the return plumbing of the main system. As described, you have the radiant plumbed in series with a radiator- the head represented by the radiant is probably at least an order of magnitude higher than the simple radiator alone. Depending on how the rest of the system is plumbed that would either impede the flow to the entire system to an unacceptable level or just the flow to that radiator. At the very least the radiant would need it's own circulation pump, even if you tied into the return leg of some portion of the existing system with closely spaced tees. But the low return-temp of the radiant could potentially damage the boiler with return temps in the condensing zone- 130F would be the hard lower-limit for return water on a gas-fired system on an ongoing basis (140F if oil-fired), which would require some of the near boiler plumbing to change, or you'll end up buying that new boiler in short years anyway.

And if you're going to do all of that, you might as well do it right. Paying a hydronic designer to come up with a cost-effective solution that works would probably be money well spent. Either that or take the course, do the math, and come up with your own solution.